Generation of trains of pulses with arbitrary delay and adjustable width from a petawatt-class laser

Trains of laser pulses of adjustable characteristics are of interest for controlling plasma instabilities, promoting controlled electron plasma wave growth, exciting plasma-based short wavelength radiation sources, and other applications. We demonstrate a simple and robust solution for the generation of trains of ultrashort laser pulses of arbitrary intra-pulse delay and adjustable pulse width from a petawatt-class laser. The train of pulses is created by wavefront division using an adaptive mirror consisting of an array of closely spaced hexagonal mirrors, whose relative positions are set by computer-controlled micro-actuators to yield the desired intra-pulse delays. A reduction of nearly an order of magnitude in the measured ω0/2 backscattered signal corresponding to stimulated Raman scattering demonstrates the ability of the adjustable pulse train technique in controlling electron plasma waves.